With development of the optical imaging photolithography technology, the Rayleigh resolution limit becomes a main obstacle that limits optical resolving power. Due to a limited numerical aperture (NA) for an objective lens imaging system, the resolution of the line width for the optical photolithography under a single-exposure condition is typically limited to about a quarter of the wavelength of the illumination light. The super-resolution optical imaging photolithography will be very important for improving the resolution and the lifecycle of the optical photolithography technology. Further, it is possible to provide a novel nano fabrication approach at low cost and high resolution for the field of micro-nano scale technology and industry.
The Superlens imaging technology based on the surface plasmon (SP) effect is a novel super-resolution optical imaging method that is drawing more attention recently. It originated from the perfect lens with the negative refractive index proposed by Prof. Pendry, Imperial College, U.K., (Pendry J B, Negative Refraction Makes a Perfect Lens, Phys. Rev. Lett. 85, 3966-3969 (2000)). A perfect lens with the negative permittivity and permeability can amplify evanescent wave components which carry sub-wavelength structural information of an object. In this way, all wave vector components could arrive at an image plane and thus participate in imaging without loss, and therefore perfect imaging without aberration and resolution limit can be achieved theoretically. However, there is no negative refraction material in the nature world. By using a metal film with negative permittivity, SPs could be excited under an incident light in a Traverse Magnetic (TM) polarization, thereby achieving the super-resolution imaging effect on both sides of the metal film.
In 2005, researchers from UC Berkeley achieved a photolithography result, with 60 nm half pitch resolution, by using a normally incident i-line illumination light from a mercury lamp (with a central wavelength of 365 nm) as a light source, and providing a PMMA (PolyMethylMethAcrylate) dielectric layer with a thickness of 40 nm, a thick metallic silver film with a thickness of 35 nm, and a photoresist photosensitive layer on a 50 nm thick chromium mask layer with a nano pattern (Fang N, Lee H, Sun C, Zhang X, Sub-diffraction-limited optical imaging with a silver superlens, Science 308: 534-537 (2005)).
On the other hand, the nano optical photolithography can also be achieved by using the local field enhancement effect and short wavelength interference effect of SPs.
Essentially, the SP imaging photolithography belongs to the near-field photolithography. The Canon company, Japan, proposes a near-field nano optical photolithography tool, in which a deformable film of SiN is used as a carrier for mask pattern, and a tight contact between the mask pattern and an upper surface of photoresist on a silicon wafer is achieved by vacuum pressure. In this way, a pattern with line-width resolving power of 32 nm or below is achieved. The main difficulty faced by the nano optical photolithography based on superlens is that the air distance between the mask structure and the photolithography substrate structure (such as, a silicon wafer) is very short. For example, the super resolution imaging technology which uses the superlens to achieve 50 nm line width resolution or below allows an working distance of only several nano-meters, very close to zero. As a result, all existing super resolution imaging technologies based on superlens adopt contacting mode in photolithography processes. In other words, the mask structure is physically contacted with the resist layer on a wafer. Obviously, such contacting mode would cause damages to the masks. It is well known that the mask with precision pattern structure is usually expensive. In order to maintain a certain lifecycle of the mask, it is desirable to extend the distance between the mask structure and the photolithography substrate structure as large as possible to prevent physically touching to each other, under the premise that the imaging resolving power is maintained. In view of the current high-precision processing level for optical planes, the attainable flatness of a device surface within a small area (for example, within a diameter of 10 mm) may be typically controlled within about 10 nm. For this reason, it is desirable to extend the distance between the mask structure and the photolithography substrate structure, for example, beyond several tens of nano-meters, to achieve a separation of these two structures. This provides an important and possible solution to achieve a contactless super-resolution imaging optical photolithography. Thus, it is desirable to design a novel super resolution imaging photolithography structure for improving the resolution of the imaging photolithography, the image contrast, and the focus depth in case of a relatively large air distance.